The dispute began after bosses at Lindsey oil refinery brought in foreign contractors
About 400 workers at Longannet power station in Fife have voted to continue their walkout over foreign contractors.
The action is in support of staff at the Lindsey oil plant in Lincolnshire who walked out over moves to hire overseas staff on a new £200m plant.
The Longannet workers met on Tuesday morning and decided to continue their unofficial action until at least Friday when they will meet again.
About 500 protesting contractors at Grangemouth are due to return to work.
On Monday, contractors at the Grangemouth refinery complex, Longnannet and Mossmorran all walked out in support of their Lincolnshire colleagues.
Unofficial strikes
More than 1,500 Scottish workers staged unofficial walkouts on Friday over the issue.
Prime Minister Gordon Brown has said the wildcat strikes were "not defensible" and Scottish Secretary Jim Murphy called on unions not to "escalate" the dispute.
The government has said it might challenge EU law to stop cheap foreign labour "undercutting" British workers.
About 130 workers at Scottish Power's Cockenzie power station are also taking part in the action, along with 80 contractors at ExxonMobil's petrochemicals plant in Mossmorran and 150 workers at the Shell plant.
Scottish Power said none of its workers was involved and the walkout by contract staff was not affecting power generation.
Shell said that the staff taking part in the action were non-operational, and that its plant was functioning as normal.
A spokesman said it understood workers at Shell's St Fergus plant in Aberdeenshire, who walked out on Friday, had gone back to work on Monday.
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